Retroreflective articles that rely upon cube corner retroreflective elements have gained wide acceptance in applications relating to traffic and personal safety marking. Cube corner retroreflective sheeting is widely used to enhance the visibility, or conspicuity, of road signs in poor lighting conditions and at night. Cube corner retroreflective sheeting has also gained wide acceptance in vehicle conspicuity marking related applications. For example, in the United States, government regulations require retroreflective materials to be positioned on semi-truck trailers to improve the conspicuity of these vehicles. Other applications for cube corner retroreflective sheeting include retroreflective sheeting for use in high-visibility clothing.
The basic cube corner retroreflective element is well known in the retroreflective arts. This element is generally a trihedral structure having three mutually substantially perpendicular lateral faces which intersect at a single reference point, or apex, and a base triangle opposite the apex. The symmetry axis, or optical axis of the element is the axis which extends through the cube apex and trisects the internal space of the cube corner element. In operation, light incident upon the base of the cube corner element is reflected from each of the three lateral faces and is redirected toward the light source. Reflection from the lateral cube corner faces may be achieved through specular reflection, in which case the lateral faces of a cube corner element are coated with a specularly reflective substance such as, for example, aluminum or silver. Alternatively, reflection may be achieved pursuant to principles of total internal reflection, in which case the faces of the cube corner element are not coated with a specularly reflective material. Retroreflective sheeting generally incorporates a structured surface including at least one array of cube corner reflective elements to enhance the visibility of an object. The total light retroreflected by the sheeting is the sum of the light retroreflected by the individual cube corner elements.
The term `entrance angularity` is commonly used to describe the retroreflective performance of retroreflective sheeting as a function of the entrance angle of light incident on the sheeting and the orientation of the sheeting. The entrance angle of incident light is typically measured with respect to an axis that extends normal to the base surface of the sheeting. The retroreflective performance of an article may be expressed as a percentage of the total light incident on the face of the article which is returned by the article at a particular entrance angle.
Conventional truncated cube corner retroreflective elements exhibit poor entrance angularity. The amount of light retroreflected by a conventional cube-corner element drops sharply when the entrance angle of incident light deviates from the optical axis of the element. Similarly, retroreflective sheeting which employs non-canted, truncated cube corner elements exhibits poor retroreflective performance in response to light which is incident upon the sheeting at high entrance angles.
Many applications could benefit from retroreflective sheeting that exhibits broad entrance angularity in multiple planes. One such application relates to retroreflective conspicuity sheeting for the trucking industry. Truck conspicuity sheeting is typically placed on the rear and the sides of truck trailers in both a horizontal orientation and a vertical orientation relative to the frame of the trailer. To function effectively, the sheeting must retroreflect light incident on the trailer at high entrance angles when the sheeting is positioned in either orientation. Accordingly, it would be desirable to provide retroreflective truck conspicuity sheeting which exhibits broad entrance angularity in two planes. Signing applications would also benefit from retroreflective sheeting having broad entrance angularity in multiple planes. In particular, retroreflective sheeting having multiple planes of broad entrance angularity reduces the importance of positioning sheeting at a particular orientation on the sign.
One method of producing retroreflective article having broad entrance angularity in multiple planes, commonly known in the art as `tiling`, involves arranging a plurality of discrete tiles of canted cube corner arrays at different orientations on the sheeting. Examples of publications relating to tiling include Tiling has the advantage of effectively producing an article with multiple planes of broad entrance angularity. However, tiling has the inherent disadvantage that, at any given orientation, only a fraction of the tiled sections are oriented to retroreflect the maximum amount of light incident on their surface. As a result, tiled cube corner sheeting suffers an inherent loss in brightness at any given orientation to gain multiple planes of entrance angularity.
U.S. Pat. No. 4,588,258 discloses a retroreflective article which has two planes of broad entrance angularity: a first plane which is substantially coincident with the plane which includes the optical axes of the cube corner elements and a second plane which is perpendicular to the first plane. However, this article exhibits substantially broader entrance angularity in the first plane than in the second plane.
It would be desirable to provide a retroreflective sheeting that has two broad planes of entrance angularity which exhibit substantially similar retroreflective performance at non-zero entrance angles. It would be ever more desirable to provide a sheeting which could achieve this optical property without sacrificing brightness, as required by tiled cube corner sheeting. The art neither discloses nor suggests such an article or a manner of achieving such an optical property.